Drilling fluids used in the drilling of subterranean oil and gas wells along with other drilling fluid applications and drilling procedures are known. In rotary drilling there are a variety of functions and characteristics that are expected of drilling fluids, also known as drilling muds, or simply “muds”. The drilling fluid should carry cuttings from beneath the bit, transport them through the annulus, and allow their separation at the surface while at the same time the rotary bit is cooled and cleaned. A drilling mud is also intended to reduce friction between the drill string and the sides of the hole while maintaining the stability of uncased sections of the borehole. The drilling fluid is formulated to prevent unwanted influxes of formation fluids from permeable rocks penetrated and also often to form a thin, low permeability filter cake which temporarily seals pores, other openings and formations penetrated by the bit. The drilling fluid may also be used to collect and interpret information available from drill cuttings, cores and electrical logs. It will be appreciated that within the scope of the description and claims herein, the term “drilling fluid” also encompasses “drill-in fluids” and “completion fluids”.
Drilling fluids are typically classified according to their base fluid. In water-based muds, solid particles are suspended in water or brine. Oil can be emulsified in the water. Nonetheless, the water is the continuous phase. Brine-based drilling fluids, of course are a water-based mud (WBM) in which the aqueous component is brine. Oil-based muds (OBM) are the opposite or inverse. Solid particles are suspended in oil, and water or brine is emulsified in the oil and therefore the oil is the continuous phase. Oil-based muds can be either all-oil based or water-in-oil macroemulsions, which are also called invert emulsions. In oil-based mud the oil may consist of any oil that may include, but is not limited to, diesel, mineral oil, esters, or alpha-olefins.
It is apparent to those selecting or using a drilling fluid for oil and/or gas exploration that an essential component of a selected fluid is that it be properly balanced to achieve the necessary characteristics for the specific end application. Because drilling fluids are called upon to perform a number of tasks simultaneously, this desirable balance is not always easy to achieve.
Filter cakes are the residue deposited on a permeable medium such as a formation surface when a slurry, suspoemulsion or suspension, such as a drilling fluid, is forced against the medium under pressure. Filtrate is the liquid that passes through the medium, leaving the cake on the medium. Cake properties such as cake thickness, toughness, slickness and permeability are important because the cake that forms on permeable zones in a wellbore can contact and wedge against downhole equipment and cause stuck pipe and other drilling problems. It will be appreciated that in the context herein, the term “filter cake” includes any oil, emulsion or invert emulsion part of the filter cake, and that the filter cake is defined herein as a combination of any added solids, if any, and drilled solids with the drilling fluid. It will also be understood that the drilling fluid, e.g. OBM is concentrated at the borehole face and partially inside the formation. Further, an open hole completion is understood to be a well completion that has no liner or casing set across the reservoir formation, thus allowing the produced fluids to flow directly into the wellbore. A liner or casing may be present in other intervals, for instance between the producing interval and the surface.
When a pipe or drill string is referred to as “stuck”, there may be varying degrees of difficulty in moving or removing the drill string from the wellbore. At one extreme, “stuck pipe” may refer to an inability to move the drill string vertically in the well, although rotation may be possible, or it may be possible to rotate the pipe or lower it back into the wellbore. At the other extreme, it may not be possible to move the drill string in any way. It is not unusual for a stuck condition to start with the possibility of limited pipe rotation or restricted vertical movement, but degrade to an inability to remove the drill string at all. Often a small volume or pill of fluid is introduced into the wellbore annulus to free the differentially stuck pipe. Traditionally, oil-based mud is the spotting fluid. Speed in mixing and placing the spot or pill is very important to successfully freeing stuck pipe. These spots or pills free the stuck pipe by covering or coating the region. It is believed to break up the filter cake, allowing the pill or spot to migrate into cracks in the filter cake and between the drill string and the cake, reducing the size of the stuck area and permitting the pipe to be pulled free.
Many operators are interested avoiding stuck pipe situations, but since stuck pipe often occurs, they also need to have ready compositions and methods for releasing stuck pipe. It would be desirable if compositions and methods could be devised to aid and improve the ability to release stuck pipe, and to release it more quickly and completely, without disadvantages.